The present invention provides zinc oxide crystal growth substrates.
Zinc oxide (ZnO) is a wide band gap semiconductor with potential for use in electrically excited devices such as light emitting devices (LEDs), laser diodes (LDs), field effect transistors (FETs), photodetectors operating in the ultraviolet and at blue wavelengths of the visible spectrum, and other similar devices. Currently gallium nitride (GaN), heteroepitaxially deposited on a single crystal substrate such as sapphire or silicon carbide, is used in radiation detecting and light emitting devices for these purposes.
Some characteristic parameters of GaN and the substrates used for its growth are shown in Table 1.
TABLE 1ThermalLatticeExpansionLatticeParametersCoefficientCrystalType[nm][106/K]GaNhexagonal (wurtzite)a = 0.31895.59c = 0.51853.17Alpha-Al2O3hexagonal (wurtzite)a = 0.47587.5c = 1.29918.56H—SiChexagonal (sphalerite)a = 0.30814.2c = 1.5114.68NdGaO3Orthorhombica = 0.542211.9b = 0.54996.6c = 0.77085.8YSZCubic (fluorite)a = 0.51511.4SiCubic silicona = 0.34312.44ZnOhexagonal (wurtzite)a = 0.32522.9c = 0.52134.75
Due to differences in crystal lattice parameters and coefficient of thermal expansion, a high density of crystal lattice dislocations is often introduced into the GaN crystal. This introduces a large lattice strain, which may create a nucleation site that may propagate lattice defects into the growing layer, and ultimately degrade the optical properties of the GaN and result in inferior device performance. A buffer layer may be placed between the GaN and the substrate, but it does not eliminate the defect migration into the active layer during crystal growth. Moreover, the buffer layer is generally deposited on a conventional self supporting crystal growth substrate such as sapphire (Al2O3) or SiC. The sapphire and silicon carbide substrates are relatively expensive. It would be an advancement in the art to provide a low cost alternative substrate for GaN crystal growth, or other active crystals.
From Table 1, ZnO is a good substrate for the crystal growth of GaN. Zinc oxide has been grown as a single crystal boule, sliced into thin wafer self supporting substrate, and used as a crystal growth substrate. This is a complicated and expensive process. It would be an advancement in the art to provide an efficient and economical method of preparing a ZnO crystal growth substrate.